Process for the production of composite sheets

ABSTRACT

THE INVENTION RELATES TO THE PLATING OF SHEETS OF FERROUS METAL WITH ALUMINUM HAVING A THICKNESS LESS THAN 0.5 MM. BY CO-ROLLING, WHEREIN THE COLD WORKED SHEETS ARE SUBJECTED TO A RESIDENCE TIME OF AT LEAST 1 HOUR IN A TEMPERATURE ZONE OF FROM 275 TO 460C. TO AVOID THE APPEARANCE OF INTERMETALLIC COMPOUNDS OF THE AL3FE TYPE.

United States Patent US. Cl. 148-115 A 7 Claims ABSTRACT OF THE DISCLOSURE The invention relates to the plating of sheets of ferrous metal with aluminum having a thickness less than 0.5 mm. by co-rolling, wherein the cold worked sheets are subjected to a residence time of at least 1 hour in a temperature zone of from 275 to 460 C. to avoid the appearance of intermetallic compounds of the AlgFC type.

This invention relates to composite sheets of ferrous metal and aluminum or with aluminum alloy.

The plating of sheets of steel with sheets of aluminum by co-rolling is normally completed by a final annealing treatment carried out at temperatures of from 480 to 550 C. in order to soften the assembly for subsequent shaping operations.

When carried out on composite sheets formed by a cold working ratio (reduction in thicknesszinitial thickness) in excess of a limit of around 90%, this annealing treatment is accompanied by the formation of intermetallic compounds of the Al Fe type along the interface and in the aluminum component. The presence of such intermetallic compounds makes the assembly fragile and, as a result, complicates subsequent shaping operations. If the intermetallic compounds come to the surface, as when the aluminum plating is thin, black-colored streaks appear, accompanied by undesirable porosity.

Several processes have been proposed with a view to eliminating or at least minimizing these phenomena, including in particular the following processes:

In one of these known processes, an annealing treatment is carried out at the lowest possible temperature and for the shortest possible time. Temperatures as low as 204 C. and annealing times as short as 2 minutes have been proposed. It is possible in this way to minimize or even completely to eliminate the appearance of intermetallic compounds, but, unfortunately, the annealing treatment carried out in this way does not result in adequate softening and subsequent shaping operations are limited as a result.

In another process, a certain quantity of silicon or of beryllium is added to the aluminum. Although this method reduces the formation of intermetallic compounds, their formation is not eliminated and, as a result, is only a partial solution to the problem. In addition, it limits the choice of the aluminum alloy to be plated.

Another process utilizes steels in which the carbon and deoxidizing agent content are as low as possible to enable the annealing treatment to be carried out at temperatures below 525 C. However, the solution which this process provides is inadequate in cases where the cold working ratios are high (British patent specification 762,817).

In another known process, utilization is made of an intermediate layer of another metal such as Cu or Ag. Although the solution which this process provides is undoubtedly effective, it is nevertheless expensive and complicated.

It is an object of this invention to produce and to provide a method for producing metal composites of the type described in which the disadvantages of the conventional processes are overcome.

In accordance with the practice of this invention, the composite sheet is subjected, before annealing, to a pretreatment under time and temperature conditions which will hereinafter be described.

In the course of the investigations which led to the present invention, it was surprisingly found that, when a composite sheet of ferrous metal covered with aluminum in the cold worked state is subjected to temperatures in a critical zone of from 275 to 460 C. for a suitable period of greater than 1 hour, a definite improvement is obtained. Irrespective of the subsequent annealing treatments, the appearance of intermetallic compounds is either completely eliminated or greatly reduced, provided, of course, that the composite sheet is not subjected to further rolling prior to annealing.

It has been found that the heat treatments which the cold worked sheet undergoes, before and after its passage through the critical temperature zone, have little influence upon the appearance of intermetallic compounds. There are in particular two practical industrial processes by which the product can be kept in the critical zone in accordance with the invention. The product can be kept for a sufiiciently long period in a furnace kept at a suitable constant temperature, being subsequently removed from the furnace for introduction into the annealing furnace after a more or less long time interval, or alternatively the product can be directly introduced into the annealing furnace which is either cold or at a temperature below the critical zone and the rate of temperature increase regulated in such a Way that the product remains in the critical zone for a sufficient period of time. For equal residence times in the critical zone, these two processes give similar results insofar as the effect on the presence of intermetallic compounds.

The conditions of the critical zone treatment are governed by the cold working ratio of the composite sheet.

For cold working ratios of from to 93%, all the temperatures in the critical zone between 275 and 460 C. are substantially equivalent and a residence time of 1 hour, at one or other of the temperatures in this zone, results in complete disappearance of the intermetallic compounds after annealing. For cold working rates in xcess of 93 the critical zone temperature narrows to within the range of 375 to 425 C. for elimination of the intermetallic compounds. In this case, however, residence periods at the other temperatures within the critical zone of 275 to 460 C. will bring about a certain reduction in the proportion of intermetallic compounds.

The exact mechanism of the reaction is not fully understood. It is believed that mixed oxide compounds of iron and aluminum are formed in the critical temperature zone at the interface between the iron and the aluminum. These compounds, the formation of which is more difficult, the greater the cold working rate of the composite sheet, prevent the iron from subsequently diffusing into the aluminum and, hence, prevent the formation of undesirable intermetallic compounds between these elements.

By way of example and in order to illustrate the invention, tests were conducted with various composite sheets consisting of a sheet of soft steel (carbon content less than 0.08%, Mn content between 0.25 and 0.40%) covered on both surfaces by a sheet of aluminum containing 1.2% Si. The composite sheets had been obtained by cold co-rolling but had not been annealed. The thickness of the layers of aluminum on the finished products amounted to between 10 and 15 microns, the thickness of the sheet of soft steel being variable. The thicknesses and the cold working ratios are set out in the following table:

Thickness (microns) of the composite Cold-working sheet ratio, percent When subjected to annealing for 2 hours at 500 C., by introduction without any particular precautions into a furnace kept at a constant temperature, these sheets gave the following result:

Appearance of Product: intermetallic compounds A None B Average C Abundant D Abundant PRODUCT C.-ISOTHERMAL DWELL TESTS Appearance of interrnetallic compounds Dwell time (hours):

Streng.-. Strong-... Strong Strong.. Slight"--. Slight.

do "do..."

The word abundan in the column headed Appearance of intermetallic compounds means that these compounds have largely broken through to the surface of the composite sheet, having passed through the layer of aluminum.

The word average in the same column means that these compounds have locally broken through to the Surface. The Word none means that these compounds cannot be detected either visually or under a microscope on a polished section transverse to the surface of the material.

It can be seen that, so far as sheet A is concerned, pretreatment of any kind is of no value.

The other sheets were subjected to various pre-treatmerits in accordance with the invention and otherwise, comprising:

either keeping the test specimens under isothermal conditions for different periods at different temperatures, and subsequently transferring the test specimens to another furnace in which they are subjected for 2 hours at 500 C. to a treatment similar to that described above,

or subjecting the test specimens to progressive increases in temperature up to the annealing temperature at different heating rates, followed without intermediate cooling by a residence time of 2 hours at 500 C. After annealing for 2 hours at 500 C., the test speci mens are examined for the appearance of intermetallic compounds.

The results obtained are set out in the following tables:

PRODUCT B.-ISOTHERMAL DWELL TESTS Average. Average..- Ncne.--..- 0. d@ dn ,(ln n do do "do..-" Sliggt do. Do

do Average.

Residence time between- Appearance of 275 and 460 375 and425 intermetallic 0., hr. hr. compounds Heating rate: 50 25 7. 4 2 Average.

18. 5 Slight.

37 Very slight.

Examination of these tables clearly shows that, with an equal residence time in the zone from 275 to 460 C. or in the zone from 375 to 425 C., the isothermal dwell treatments or the constant heating rate treatments give equivalent results insofar as the appearance of intermetallic compounds during annealing.

If cold working makes it necessary to carry out a pretreatment of long duration, it is preferred to apply isothermal dwell treatments using either a two-compartment furnace or two separate furnaces.

If, by contrast, the pro-treatment can be relatively short, it is of greater advantage suitably to select the Appearance of intermetallic compounds Dwell time (hours):

s2 do .410 :do .do

None Slight.

heating rate using one and the same furnace for the pretreatment and for annealing.

In addition, consideration of the result of the tables relating to those tests where pre-treatment is carried out by regulating the heating rate before annealing, shows that the maximum rate required to obtain a product without any intermetallic compounds is lower, the greater the cold working of the product. This maximum rate V can be expressed in accordance with the following equation:

log V=580.6e

where e represents the cold working ratio in percent.

It will be understood tht changes may be made in the details of formulation and operation without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. A process for the production of composite sheets with a thickness of less than 0.50 millimeter comprising a covering of aluminum or alloy of aluminum on a substrate in which at least the portion in contact with the covering is a ferrous metal selected from the group consisting of iron and soft steel formed by cold working to a reduction ratio of the composite sheet of more than 90%, followed by annealing at a temperature of at least 480 0, wherein the composit sheet is subjected before annealing to a pre-treatment comprising a residence time of at least one hour at a temperature within the range of 275 to 460 C.

2. A process as claimed in claim 1 in which the cold working is by cold rolling.

3. A process as claimed in claim 1 in which the cold 6 working ratio is in excess of 93% and in which the pretreatment comprises a residence time of at least one hour at a temperature within the range of 375 to 425 C.

4. A process as claimed in claim 1 in which the pretreatment is carried out by maintaining an average heating rate of less than 100 C. per hour while in the temperature range of 275 to 460 C. during the heating stage which precedes annealing.

5. A process as claimed in claim 3 in which the composite sheet passes through the temperature range of 375 to 425 C. at an average rate V below that expressed in accordance with the equation: log V=58-0.6e.

6. A process as claimed in claim 1 in which the pretreatment is an isothermal treatment.

7. A composite sheet of aluminum or alloy of aluminum on a ferrous metal produced by the process of claim 1.

References Cited UNITED STATES PATENTS 2,809,422 10/1957 Schultz 29497.5 2,818,360 12/1957 Porter 148l2.1 2,965,963 12/1960 Batz et al. 29487 3,248,270 4/1966 Laidman et al. 148l2 3,300,837 1/1967 Fisher et al. 29149.5 3,300,838 1/1967 Slater et al. 29149.5 3,400,450 9/1968 Nock, Jr. et al. 29497.5

WAYLAND W. STALLARD, Primary Examiner 

